Purification of inert gases to remove trace impurities

ABSTRACT

An argon purification system is provided which includes a cryogenic heat exchanger, a cryogenic distillation column. The cryogenic heat exchanger is configured to remove heat from a pre-treated argon waste stream to create a cold feed stream. The cryogenic distillation column includes packing, a reboiler, and an overhead condenser, as well as an upper portion and a lower portion and is configured to receive a liquid feed stream and to produce a bottoms argon product stream and a gas waste stream. The reboiler is positioned in the lower portion of the cryogenic distillation column and is configured to condense the cold feed stream to produce the liquid feed stream. The condenser is positioned in the upper portion of the cryogenic distillation column and is configured to heat the bottoms argon product stream such that the bottoms argon product stream evaporates to a purified vapor phase argon stream.

TECHNICAL FIELD OF INVENTION

The present invention relates to a system for the purification of inertgases. More specifically, the present disclosure relates to a system forthe onsite removal of trace impurities, such as nitrogen and methane,from an argon stream using cryogenic distillation separation.

BACKGROUND OF THE INVENTION

The use of argon as an inert gas is important to a number of industriesincluding the production of monocrystalline and polycrystalline silicon,steel making (e.g., using the hot isotatic pressing (HIP) process tomake steel, aluminum and nonmetallic castings from powders), heattreating, and semiconductor, wafer, and electronics manufacturing. Giventhe rising costs of argon gas, onsite purification of waste argonstreams to recycle back to the manufacturing process is increasinglycommon.

One manufacturing process suitable for argon purification and recycle isthe production of monocrystalline silicon using the Czochralski Process.High purity argon, i.e., having a purity of 99.999% or greater, is usedin the process to control the atmosphere and to purge contaminants andvolatilized materials from the process as a waste stream. Purity of theargon stream is critical because it affects the both the purity and thequality of the silicon ingot grown in the Czochralski Process.

The waste stream from the process, which may include between about 95 toabout 99% argon (Ar), may contain any number of contaminants including:oxygen (O₂), hydrogen (H₂), volatilized silicon oxide (SiO), carbondioxide (CO₂), carbon monoxide (CO), nitrogen (N₂), water (H₂O), methane(CH₄), other hydrocarbons, and other impurities. Although commercialtechnologies currently address the removal of contaminants such as O₂,H₂, CO₂, CO, H₂O, and hydrocarbons, cryogenic distillation is still thepreferred route for the removal of N₂ and CH₄.

Current methods for the purification of argon typically utilize acryogenic distillation process used in conjunction with a pre-treatmentprocess. Current pre-treatment process systems begin by compressing thewaste stream. The compressed gas is fed, along with excess O₂ and H₂ ifnecessary, to a series of commercially available catalyst beds thatoxidize CO and CH₄ to CO₂ and H₂O. The CO₂ and H₂O can be removed byadsorption, either by using molecular-sieve or a combination of silicagel and molecular-sieve. The waste stream now contains primarily argon,N₂, and H₂, and may also contain minute quantities of CH₄ (methaneslippage), which must be removed prior to recycle to the manufacturingprocess. With the CO₂ and H₂O removed, the argon waste stream may be fedto a cryogenic distillation system capable of removing the N₂, anyremaining H₂.

Argon purification systems, as described above, are available inpackaged skids typically sold to companies for housing on their propertyfor use in their manufacturing process. However, current systems do notaddress methane slippage and are incapable of removing impurities belowpart per million (ppm) levels.

Therefore, it would be beneficial to have a simpler cryogenicdistillation system with fewer mechanical parts. Additional benefitscould be gained if a simpler system was capable of removing moreimpurities and addressing methane slippage from the pre-treatmentprocess.

SUMMARY OF THE INVENTION

The present invention relates to a system for the purification of inertgases. More specifically, the present disclosure relates to a system forthe onsite removal of trace impurities, such as nitrogen and methane,from an argon stream using cryogenic distillation separation.

In one embodiment, an argon purification system is provided. The argonpurification system includes a cryogenic heat exchanger configured toremove heat from a pre-treated argon waste stream to create a cold feedstream. A cryogenic distillation column includes a reboiler and anoverhead condenser, said column having an upper portion and a lowerportion and is configured to receive a liquid feed stream and to producea bottoms argon product stream and a gas waste stream. The reboiler ispositioned in the lower portion of the cryogenic distillation column andis configured to remove heat from the cold feed stream such that thecold feed stream condenses to the liquid feed stream. The condenser ispositioned in the upper portion of the cryogenic distillation column andis configured to heat the bottoms argon product stream such that thebottoms argon product stream evaporates to a purified vapor phase argonstream.

In certain embodiments, the cryogenic heat exchanger heats the purifiedvapor phase argon stream with the heat removed from the pre-treatedargon waste stream. In certain embodiments, the cryogenic heat exchangerheats the gas waste stream with the heat removed from the pre-treatedargon waste stream. In certain embodiments, the reboiler uses the heatremoved from a cryogenic distillation column overheads to heat thebottoms argon product stream. In certain embodiments, the overheadcondenser uses heat removed from the cryogenic distillation columnoverheads to heat the bottoms argon product stream. In certainembodiments, the bottoms argon product stream undergoes expansion in anexpansion device before being fed to the overhead condenser, such thatthe temperature of the bottoms argon product stream is reduced in theexpansion device. In certain embodiments, a pump provides the drivingforce to get the bottoms argon product stream to the overhead condenser.

In certain embodiments, the cryogenic distillation column is configuredwith additional stages above the overhead condenser. In certainembodiments, the argon purification system includes a sorbent bedsystem, such that the sorbent bed system accepts the bottoms argonproduct stream, removes contaminants from the bottoms argon productstream, and then feeds a sorbent bed outlet stream to the overheadcondenser. In certain embodiments, the sorbent bed system is filled withregenerable adsorbents. In certain embodiments, the sorbent bed systemis filled with getter materials.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, claims, and accompanying drawings. It is to be noted,however, that the drawings illustrate only several embodiments of theinvention and are therefore not to be considered limiting of theinvention's scope as it can admit to other equally effectiveembodiments.

FIG. 1 shows one preferred embodiment of the invention.

FIG. 2 shows another preferred embodiment of the invention.

FIG. 3 shows an alternative preferred embodiment of the invention.

FIG. 4 shows another preferred embodiment of the invention.

DETAILED DESCRIPTION

While the invention will be described with several embodiments, it isunderstood that one of ordinary skill in the relevant art willappreciate that many examples variations, and alterations to thefollowing details are within the scope and spirit of the invention.Accordingly, the exemplary embodiments of the invention described hereinare set forth without any loss of generality, and without imposinglimitations, relating to the claimed invention.

In one embodiment, the present invention describes a cryogenicdistillation system capable of removing trace impurities from an argonstream.

FIG. 1 provides an illustration of an embodiment of the presentinvention. Pre-treated argon waste stream 2 is the outlet of thepre-treatment process. The waste gas stream from the manufacturingprocess will include argon, H₂, O₂, CO₂, CO, CH₄, and H₂O, along withother trace impurities. The pre-treatment process begins by compressingthe waste gas stream. The compressed gas is then fed, along with excessO₂ and H₂, or air, if necessary, to a series of commercially availablecatalyst beds that oxidize CO, CH₄, and O₂ to CO₂ and H₂O. The CO₂ andH₂O are removed by adsorption, either by using molecular-sieve or acombination of silica gel and molecular-sieve, resulting in pre-treatedargon waste gas stream 2.

Pre-treated argon waste stream 2 contains greater than about 95% argon,less than about 4% N₂, less than about 1% H₂, less than about 1 ppm H₂,O₂, CO, and CH₄, and no CO₂ or H₂O. The exact temperature, pressure andcomposition of pre-treated argon waste stream 2 will depend on thespecific manufacturing and pre-treatment processes that are employed. Incertain embodiments, the pre-treatment process removes as much O₂, CO,and CH₄ as possible, to achieve concentrations from about 1 ppm to 10ppm, alternatively to below 1 ppm.

Pre-treated argon waste stream 2 is fed into cryogenic heat exchanger10, where it is cooled to a temperature between about −150° C. and −160°C., alternatively between about −152° C. and −157° C., alternativelybetween about −153° C. and −155° C., alternatively about −153.6° C.Cryogenic heat exchanger 10 may be any heat exchanger of the type andmaterials commonly employed in a cryogenic air separation process. Incertain embodiments, the heat exchanger is constructed of brazedaluminum.

Cold feed stream 12 exits cryogenic heat exchanger 10 and entersreboiler 30. Additional energy is removed from cold feed stream 12 inreboiler 30, and the heat from cold feed stream 12 is used to providereboil liquid at the bottom of column 20. This results in condensing ofcold feed stream 12. Liquid feed stream 32 exits reboiler 30 at atemperature between about −150° C. to about −160° C., preferably about−156.9° C. Liquid feed stream 32 is then fed to cryogenic distillationcolumn 20 at a point above the bottom section and below the uppersection, preferably in the packing section.

Argon stream 4 is fed to cryogenic distillation column 20 to make up forargon lost as waste in gaseous waste stream 26 and the liquid purge 28.Any refrigeration requirements for the process such as heat leakage,temperature differences in heat exchanger 10, liquid purge or otherlosses can be compensated by the addition of argon stream 4. In oneembodiment, Argon stream 4 is a high purity source of liquid argoncontaining at least about 99.999% argon, less than about 0.001% O₂, andless than about 0.001% N₂. Advantageously, argon feed stream 4 can alsoprovide additional cooling for cryogenic distillation column 20. Thetemperature of argon feed stream 4 is preferably close to its dew point.

Bottoms argon product stream 22 exits cryogenic distillation column 20.Bottoms argon product stream 22 has a purity of at least about 99.999%argon. After exiting cryogenic distillation column 20, bottoms argonproduct stream 22 is cooled by expansion in expansion device 50.Expansion device 50 may be any device capable of reducing the pressureand temperature of bottoms argon product stream 50. In one embodiment,bottoms argon product stream 22 is cooled to at or about its bubblepoint.

Bottoms argon product stream 22 can be supplied to overhead condenser40. In one embodiment, argon lift stream 6 (a small stream of argon gas,less than about 1% by mass of bottoms argon product stream 22) can beadded to bottoms argon product stream 22 after expansion valve 50 as alift gas to drive the stream to overhead condenser 40. This feature canbe useful to ease the liquid transfer when distillation column 20 istall, particularly due to the relatively high density of liquid argonand the low differential pressure between the bottom of distillationcolumn 20 and overhead condenser 40. A partial vapor stream thatincludes argon lift stream 6 and bottoms argon product stream 22 (i.e.,bottoms lift stream 52), operates by virtue of the buoyancy of agas-liquid slug.

Bottoms lift stream 52 provides cooling to overhead condenser 40, whichvaporizes bottoms lift stream 52 to create purified vapor phase argonstream 24. Vapor from the top of cryogenic distillation column 20partially condenses in overhead condenser 40 due to the cooling providedby bottoms lift stream 52. A small liquid purge stream 28 containingsome heavy impurities such as hydrocarbons, carbon dioxide, etc. . . .can be removed from overhead condenser 40.

Purified vapor phase argon stream 24 can be fed to cryogenic heatexchanger 10 to provide cooling for pre-treated argon waste stream 2.Product stream 14, having a temperature of about ambient temperature,may be returned to the manufacturing process requiring pure argon, maybe stored, may be diverted for other processes, or may be distributedoff-site. The purity of product stream 14 is greater than about 99.999%argon.

Nitrogen and H₂, being more volatile than argon, collect near the top ofcryogenic distillation column 20 and are removed from the distillationcolumn as gaseous waste stream 26. Gaseous waste stream 26 can besupplied to cryogenic heat exchanger 10 to provide cooling ofpre-treated argon waste stream 2. Warm gas waste stream 16 may then berecycled to the manufacturing process, disposed of, or used toregenerate the mole-sieve beds of the pre-treatment process.

In a second embodiment of the system in FIG. 1, a pump (not shown) canbe used to supply the driving force to drive bottoms argon productstream 22 to overhead condenser 40. The pump may be of any type andmaterials commonly used in a cryogenic distillation system. In oneembodiment, argon lift stream 6 is not used when the pump is used.

FIG. 2 demonstrates an additional embodiment of the invention. In thisembodiment, in addition to the features shown in FIG. 1 and describedabove, additional separation stages can been included in cryogenicdistillation column 20 above overhead condenser 40. The additionalseparation stages allow for further purification of the argon stream byremoving CH₄ not removed during the pre-treatment process, thusaddressing methane slippage. Methane, which is less volatile than argon,will be carried out the bottom of cryogenic distillation column 20 inbottoms argon product stream 22. The additional stages in cryogenicdistillation column 20 allow for the separation and removal of this CH₄prior to purified vapor phase argon stream 24 exiting cryogenicdistillation column 20. Methane waste stream 28 may be bled from thecolumn below the additional stages and disposed of or redirected asnecessary. The flowrate of argon stream 4 can be adjusted to account foradditional argon losses in methane waste stream 28. It is understoodthat the additional separation stages shown in FIG. 2 can also be addedto the additional embodiments described herein, for example FIG. 3 andFIG. 4.

FIG. 3 illustrates an alternative embodiment of the invention. In thisembodiment, in addition to the features shown in FIG. 1 and describedabove, a portion of product stream 14 can be diverted to compressor 60through recycle stream 15. Compressor 60 compresses recycle stream 15 tothe pressure of cryogenic distillation column 20, creating compressedargon recycle stream 68. Compressed stream 68 can be fed to cryogenicheat exchanger 10 to be cooled. The cooled and compressed argon recyclestream 18 is cooled to a temperature at or about its dew point, and isfed to cryogenic distillation column 20. Between about 5 and 25% ofproduct stream 14 by volume can be recycled to cryogenic distillationcolumn 20 to improve the overall recovery of argon. In alternateembodiments, between about 5 and 10% by volume is recycled,alternatively between about 10 and 15% by volume, alternatively betweenabout 15 and 20% by volume, alternatively between about 20 and 25%. Incertain embodiments, by recycling approximately 10% of product stream 14to cryogenic distillation column 20 can increase overall argon recoveryfrom about 83% to greater than 90%, alternatively greater than 95%,alternatively to about 99%.

FIG. 4 illustrates yet another embodiment of the invention. In thisembodiment, in addition to the features shown in FIG. 1 and FIG. 3 anddescribed above, bottoms argon product stream 22 can be supplied tosorbent bed system 80 to reduce contaminants in bottoms argon productstream 22. Pump 70 can provide the drive necessary so sorbent bed feedstream 72 can pass through sorbent bed system 80 and can enter overheadcondenser 40.

Sorbent bed system 80 can include two or more beds in parallel, oralternatively can include two or more beds such that at least one is aback-up for use when another bed is being serviced. Sorbent bed system80 may be filled with regenerable adsorbents or nonregenerable gettermaterials. Getter materials can include finely divided metals (such as,for example, zirconium, tantalum, copper, nickel, etc.) on a substrate,such as alumina, ceria or kieselguhr. In certain embodiments, sorbentbed system 80 will remove contaminants such that the concentration ofCO, CH₄, H₂, O₂, N₂, and H₂O is reduced to between about 1 ppm to about10 ppm, alternatively to between about 1 ppb to 10 ppb. Outlet stream 82from sorbent bed system 80 may be returned to overhead condenser 40, oralternatively may be diverted for other processes.

Example 1

The present invention is further demonstrated by the followingillustrative embodiment, which does not limit the claims of the presentinvention.

Table 1 provides data for a representative process, utilizing theembodiment illustrated in FIG. 1. The manufacturing process pre-treatedargon gas waste stream is provided from a monocrystalline siliconproduction process. A molar gas flowrate of 500 m³/h was chosen forsimulation purposes. The process conditions of pre-treated argon wastestream 2 are based on the outlet of a pre-treatment process, whichtypically takes place at an ambient temperature, but with a compressedgas stream, thus for simulation purposes an inlet temperature of 20° C.and 147 psig were chosen. Finally, the composition of pre-treated argonwaste stream 2 assumes some CH₄ leakage and impurities of CO and O₂ onthe part per million scale.

TABLE 1 Representative Data Stream Name 2 12 32 22 52 24 14 26 16 VaporFraction 1 1 0 0 0.0303 1 1 1 1 Temperature (C.) 20 −153.6 −156.9 −159−161.9 −162 16.3 −159.5 16.3 Pressure (psig) 147 144.1 143.5 111.5 90.3190.02 87.12 110.5 107.6 Molar Flow (Nm3/ 500 500 500 436.7 438.7 438.7438.7 89.85 89.85 h) (gas)) Comp Mole Frac 0.995042 0.995042 0.9950420.999992 0.999992 0.999992 0.999992 0.972445 0.972445 (Ar) Comp MoleFrac 0.003960 0.003960 0.003960 0.000005 0.000005 0.000005 0.0000050.022014 0.022014 (N2) Comp Mole Frac 0.000995 0.000995 0.000995 0 0 0 00.00537 0.00537 (H2) Comp Mole Frac 0.000001 0.000001 0.000001 0.0000010.000001 0.000001 0.000001 0 0 (CH4) Comp Mole Frac 0.000001 0.0000010.000001 0 0 0 0 0.000003 0.000003 (CO) Comp Mole Frac 0.000001 0.0000010.000001 0.000001 0.000001 0.000001 0.000001 0.000001 0.000001 (O2) CompMole Frac 0 0 0 0 0 0 0 0 0 (CO2) Comp Mole Frac 0 0 0 0 0 0 0 0 0 (H2O)

As shown in Table 1, the present invention is capable of removing traceimpurities to produce an argon stream (product stream 14), having anargon purity of greater than 99.999%. Cold feed stream 12 leavingcryogenic heat exchanger 10 was cooled to a temperature of −153.6° C. byusing the cold from purified vapor phase argon stream 24 and gaseouswaste stream 26.

While the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing description. Accordingly, it is intendedto embrace all such alternatives, modifications, and variations as fallwithin the spirit and broad scope of the appended claims. The presentinvention may suitably comprise, consist or consist essentially of theelements disclosed and may be practiced in the absence of an element notdisclosed. Furthermore, if there is language referring to order, such asfirst and second, it should be understood in an exemplary sense and notin a limiting sense. For example, it can be recognized by those skilledin the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unlessthe context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means thesubsequently identified claim elements are a nonexclusive listing (i.e.,anything else may be additionally included and remain within the scopeof “comprising”). “Comprising” as used herein may be replaced by themore limited transitional terms “consisting essentially of” and“consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, makingavailable, or preparing something. The step may be performed by anyactor in the absence of express language in the claim to the contrary arange is expressed, it is to be understood that another embodiment isfrom the one.

Optional or optionally means that the subsequently described event orcircumstances may or may not occur. The description includes instanceswhere the event or circumstance occurs and instances where it does notoccur.

Ranges may be expressed herein as from about one particular value,and/or to about another particular value. When such particular valueand/or to the other particular value, along with all combinations withinsaid range.

All references identified herein are each hereby incorporated byreference into this application in their entireties, as well as for thespecific information for which each is cited.

What is claimed is:
 1. An argon purification system, the systemcomprising: a cryogenic heat exchanger, wherein the cryogenic heatexchanger is configured to remove heat from a pre-treated argon wastestream to create a cold feed stream; a cryogenic distillation columncomprising a packing section, a reboiler, and an overhead condenser,said cryogenic distillation column having an upper portion and a lowerportion and being configured to receive a liquid feed stream and toproduce a bottoms argon product stream and a gas waste stream; whereinthe reboiler is positioned in the lower portion of the cryogenicdistillation column and is configured to remove heat from the cold feedstream such that the cold feed stream condenses to the liquid feedstream; and wherein the overhead condenser is positioned in the upperportion of the cryogenic distillation column and is configured to heatthe bottoms argon product stream such that the bottoms argon productstream evaporates to a purified vapor phase argon stream.
 2. The systemas claimed in claim 1, wherein the cryogenic heat exchanger is furtherconfigured to heat the purified vapor phase argon stream with the heatremoved from the pre-treated argon waste stream.
 3. The system asclaimed in claim 1, wherein the cryogenic heat exchanger is furtherconfigured to heat the gas waste stream with the heat removed from thepre-treated argon waste stream.
 4. The system as claimed in claim 1,wherein the reboiler is configured to use the heat removed from the coldfeed stream to boil the liquid in a cryogenic distillation columnbottoms.
 5. The system as claimed in claim 1, wherein the overheadcondenser is configured to use heat removed from a cryogenicdistillation column overheads to heat the bottoms argon product stream.6. The system as claimed in claim 1, further comprising an expansiondevice in fluid communication with the bottom portion of the cryogenicdistillation column, the expansion device configured to receive andexpand the bottoms argon product stream before the bottoms argon productstream is fed to the overhead condenser, such that the temperature ofthe bottoms argon product stream is reduced.
 7. The system as claimed inclaim 1, further comprising a pump that is configured to provide adriving force to get the bottoms argon product stream to the overheadcondenser.
 8. The system as claimed in claim 1, further comprising anargon lift stream in fluid communication with the bottoms argon productstream, the argon lift stream configured to provide additional lift tothe bottoms argon product stream by introducing gaseous argon into thebottoms argon product stream.
 9. The system as claimed in claim 8,wherein the argon lift stream is less than about 1% by mass of thebottoms argon product stream.
 10. The system as claimed in claim 1,wherein the cryogenic distillation column further comprises additionalstages disposed above the overhead condenser configured to removemethane from the bottoms argon product stream.
 11. The system as claimedin claim 1, further comprising a sorbent bed system, wherein the sorbentbed system is configured to accept the bottoms argon product stream, toremove contaminants from the bottoms argon product stream to produce asorbent bed outlet stream, and then feed the sorbent bed outlet streamto the overhead condenser.
 12. The system as claimed in claim 11,wherein the sorbent bed system is filled with regenerable adsorbents.13. The system as claimed in claim 11, wherein the sorbent bed system isfilled with getter materials.
 14. The system as claimed in claim 1,wherein the pre-treated argon waste stream is sourced from amonocrystalline silicon production process.
 15. The system as claimed inclaim 1, further comprising a recycle compressor in fluid communicationwith the heat exchanger, the purified vapor phase argon stream, and thecryogenic distillation column, wherein the recycle compressor isconfigured to compress a portion of the purified vapor phase argonstream to produce a compressed argon recycle, wherein the heat exchangeris configured to cool the compressed argon recycle before introducingthe compressed argon recycle into the cryogenic distillation column forfurther purification.